Livin, my ds is one of the top readers in his year and the youngest child ever to become a free reader in his school, but, he has developed coping strategies to handle his visual issues, so he will sit with his book on the floor or sofa and squat above it, leaning with one elbow on one knee and his hand covering his eye. When he has to read aloud, he hates it, because he is made to have the book on his desk or hold it at arms length and then he skips words or lines.
Iirc, it was visual sequencing that brought his scores down for processing on the WISC IV. He was on the 16th centile for processing, but 95th for verbal reasoning. According to the EP who did the assessment and the orthoptist and consultant optician I have spoken to, this is because he struggles to track across the page in a straight line and the test called for him to constantly look from the left to right on the page and then manipulate or sequence the image he saw.
When writing, he doesn't put spaces between individual words, but will put big spaces between letters within a word. A page of his writing looks like a whole page of random letters with some placed together and lots invidually/separately placed. This doesn't happen when he touch types though, as he is typing as he thinks, rather than looking at what he's producing, iyswim. When writing, all his letters are random sizes, some capital, some lower case and placed randomly above, on or below the line.
The Alphasmart is simpler than a laptop and only shows about a paragraph/few lines of text at a time, so is better for children with visual/dyslexia type problems as it narrows the field of vision. Could a word processing programme on a laptop perhaps be too bright or too much visual information all at once for your ds to handle - much like a full page of words in a book?
He can spell long complicated words, but generally gets simple ones wrong time and again in his work and can pass his spelling tests, but can't apply those spellings to his written work.
The muscle tone problems in your ds's shoulder and pelvis will almost certainly be having an impact on his ability to sit comfortably and concentrate. My ds is now also developing a curved spine and suffers from back-ache both between his shoulders and lower down in the middle of his back, but we have been told he doesn't qualify for physio/OT in our PCT, because he tested on the cut off for dyspraxia. One point below and he'd have been offered help.
Its awful when they are so resistant to writing, yet you know they are bright, but unable to show it in academic terms, especially when they believe they aren't clever enough as a result.
It's taken us two years of softly softly with ds and on reflection the best motivator was taking the pressure off him to write for a while.
Would the school possibly consider him using voice recognition software? I know some school are a bit anti, but if its a way he could show them what he can do without the barrier and pressure of having to write, it might be worth the fight.